1. Field of the Invention
The present invention relates to a gas sensor element, and to a gas sensor including the gas sensor element.
2. Description of the Related Art
A conventionally known gas sensor is attached to an exhaust path of an internal combustion engine such as an automotive engine for detecting a concentration of oxygen or a concentration of NOx in exhaust gas (gas to be measured) (refer to Patent Documents 1 and 2). Patent Documents 1 and 2 describe a gas sensor element used in the gas sensor and configured such that a plurality of platelike solid electrolyte bodies are laminated together. The gas sensor element includes electrodes provided on the front sides or back sides of the solid electrolyte bodies, and leads provided on the front sides or back sides of the solid electrolyte bodies and connected to the electrodes.
In Patent Documents 1 and 2, in order to ensure oxygen pumping performance, the electrodes are porous; as a result, the electrodes have gas permeability and water permeability. In contrast, the leads are formed to be dense since, in order to improve electrical conductivity, high density per unit area is desired. As a result, the leads are gas-impermeable and water-impermeable.
[Patent Document 1] Japanese Patent No. 4165652
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2010-122187
3. Problems to be Solved by the Invention
FIGS. 3 and 4 in Patent Document 1 show a first electrode having a space exposure portion which is exposed to an internal space communicating with an ambient atmosphere of the gas sensor element. Furthermore, FIGS. 3 and 4 show a first lead connected to the first electrode. The first lead is connected to the first electrode within the internal space so as to overlap the first electrode. However, in the case of combining the water-permeable first electrode and the water-impermeable first lead as mentioned above, the following problem may arise. That portion (hereinafter, referred to as a first connection portion) of the first electrode which overlaps the first lead may deteriorate in oxygen pumping performance, with a resultant deterioration in accuracy in detecting a concentration of oxygen or a concentration of NOx in a gas to be measured.
In view of the above problem, there is a proposal to extend a portion of the electrode to a position which is not exposed to the internal space, in order to dispose the first connection portion at a position not exposed to the internal space. This can restrain deterioration in accuracy in detecting a concentration of oxygen or a concentration of NOx in gas to be measured, as a result of deterioration in oxygen pumping performance stemming from disposition of the first connection portion within the internal space.
However, even though the first connection portion is disposed at a position not exposed to the internal space, the following problem may arise. For example, in winter, water may condense on the surface of the gas sensor element. Water formed through condensation (condensed water) may penetrate into the internal space of the gas sensor element and further into the water-permeable first electrode through the space exposure portion. Water which has penetrated into the first electrode reaches the first connection portion connected to the first lead. Since the first lead is water-impermeable, water fails to penetrate into the first lead; as a result, water may stagnate in the first connection portion. When condensed water stagnating in the first connection portion freezes and thus expands in volume, stress is generated in such a direction as to separate layers between which the first connection portion is sandwiched, potentially generating a crack between the layers.
The present invention has been made to solve the above problems, and an object thereof is to provide a gas sensor element which has water-permeable electrodes, and water-impermeable leads connected to the electrodes and which, even when water penetrates into the water-permeable electrodes, can prevent generation of cracks therein without deterioration in oxygen pumping performance of the electrodes, as well as a gas sensor including the gas sensor element.